What is the meaning of maincategorie / subcategorie / subcategory of subcategory ?

Fastpublish's left and right navigation offers three hierarchically sorted levels:

1. maincategory: The main categories (underlined and in bold-font) are heading the subcategories. When a main category is selected, an overview of all related sub- and sub-subcategories appears in the center block. 
2. subcategory:
   (this is an article directly related to its main category [single-indented]): When a subcategory is selected the navigation will show:
3. subcategory of subcategory (this is an article directly related to its subcategory [double-indented]):
   Note: A double-indented subcategory of a subcategory is only visible in the navigation when a subcategory is selected.
   
   The following illustration shows the navigation of this online help